In December 2006, the Orange County Health Care Agency (OCHCA) and California Department of Health Services (CDHS) were notified of two potential cases of foodborne botulism in an older Asian couple. This report summarizes the subsequent investigation, which identified home-prepared fermented tofu (soybean curd) as the source. The public should be aware of the risk for botulism when preparing fermented tofu at home.

Botulism is a toxin-induced paralytic illness characterized by cranial nerve palsies and descending flaccid paralysis. Treatment is based on supportive care and administration of botulinum antitoxin; recovery can take from weeks to months. Foodborne botulism results from eating foods containing botulinum toxin.1 Although rare, foodborne botulism is a public health emergency because of the potential severity of illness and exposure of many persons to contaminated food.

On November 28, 2006, a woman aged 67 years had onset of double vision, followed the next day by bilateral ptosis. An ophthalmologist attributed these symptoms to long-standing diabetes mellitus. On December 4, she visited her primary-care physician because of double vision, ptosis, dizziness, difficulty swallowing, slurred speech, drooling, and right arm weakness. Physical examination revealed limitation of upward gaze, bilateral ptosis, sluggish tongue movement, and mild right upper extremity weakness.

The woman's husband, aged 75 years, reported 3 days of worsening double vision, dizziness, and difficulty swallowing. On physical examination, he also had mild right ptosis and sluggish tongue movement.

Both patients were admitted to an intensive care unit. On December 5, physicians suspected foodborne botulism, notified OCHCA, and collected clinical specimens for testing. CDHS dispatched botulinum antitoxin to the hospital, and it was administered to the couple. Both patients were hospitalized for more than 1 week with no further symptom progression. Botulinum toxin was not detected in serum or stool samples from the patients. However, Clostridium botulinum type A was detected in enrichment cultures of the stool samples of both patients. Both patients have some blurred vision but otherwise have recovered.

On December 5, OCHCA visited the couple's home and identified multiple potential sources of intoxication. OCHCA interviewed the patients using photos of home-prepared food items to overcome the language barrier and identify the most suspect food. The patients reported they recently had been eating a new batch of home-prepared fermented tofu. Although both had eaten fermented tofu from this batch every day, the woman ate more than her husband. CDHS Microbial Diseases Laboratory found both C. botulinum type A and botulinum toxin type A in the fermented tofu samples, which had a pH of 6.8.

The tofu was a commercially packaged product purchased at a retail market. In the home, the tofu was boiled, towel dried, and cut into cubes. The cubes were placed in a bowl, covered with plastic wrap, and stored at room temperature for 10-15 days. The tofu was then transferred to glass jars with chili powder, salt, white cooking wine, vegetable oil, and chicken bouillon to marinate at room temperature for 2-3 more days. Finally, the fermented tofu was stored and eaten at room temperature.

C. botulinum spores exist widely in the environment, but proper food-preparation practices inhibit spore germination and toxin production.2 Environmental conditions that facilitate spore germination and growth include a pH >4.6, anaerobic conditions, low salt or sugar content, and temperatures >39.2°F (>4°C).2 In the case described in this report, the growth of C. botulinum and production of toxin might have been facilitated by several factors: (1) the almost neutral pH of the fermented tofu, (2) boiling the tofu, potentially creating an anaerobic environment, and (3) room temperature (approximately 68°F–77°F [20°C–25°C]) storage of the product for days during and after preparation.

The wife reported she has lived in the United States for more than 25 years and, during this time, has prepared fermented tofu using the same recipe she learned as a student in Taiwan. Preparation of this batch was not notably different, and the reason for contamination this time is not clear.

This is the first U.S. report of botulism caused by eating home-prepared fermented tofu. Historically, most foodborne botulism cases in the United States result from consumption of improperly prepared home-canned foods.1 However, fermented foods, including fish, seal, and whale, also have been associated with botulism. Fermented tofu is popular in Asia, and homemade fermented bean products, including tofu, are the most common foods causing botulism in China. During 1958-1989, home-fermented bean products were associated with 63% of approximately 2,000 cases of botulism in China.3 Clinicians, public health workers, and the public should be advised that home preparation of fermented tofu can result in foodborne botulism.